Modern computing and display technologies have facilitated the development of systems for so-called “virtual reality” or “augmented reality” experiences, wherein digitally reproduced images or portions thereof are presented to a user in a manner where they seem to be, or may be perceived as, real. A virtual reality (VR) scenario typically involves presentation of digital or virtual image information without transparency to other actual real-world visual input, whereas an augmented reality (AR) scenario typically involves presentation of digital or virtual image information as an augmentation to visualization of the actual world around the end user.
For example, referring to FIG. 1, an augmented reality scene 4 is depicted wherein a user of an AR technology sees a real-world park-like setting 6 featuring people, trees, buildings in the background, and a concrete platform 8. In addition to these items, the end user of the AR technology also perceives that he “sees” a robot statue 10 standing upon the real-world platform 8, and a cartoon-like avatar character 12 flying by which seems to be a personification of a bumble bee, even though these elements 10, 12 do not exist in the real world. As it turns out, the human visual perception system is very complex, and producing a VR or AR technology that facilitates a comfortable, natural-feeling, rich presentation of virtual image elements amongst other virtual or real-world imagery elements is challenging.
VR and AR systems typically employ head-worn displays (or helmet-mounted displays, or smart glasses) that are at least loosely coupled to a user's head, and thus move when the end user's head moves. If the end user's head motions are detected by the display subsystem, the data being displayed can be updated to take the change in head pose (i.e., the orientation and/or location of user's head) into account.
As an example, if a user wearing a head-worn display views a virtual representation of a three-dimensional (3D) object on the display and walks around the area where the 3D object appears, that 3D object can be re-rendered for each viewpoint, giving the end user the perception that he or she is walking around an object that occupies real space. If the head-worn display is used to present multiple objects within a virtual space (for instance, a rich virtual world), measurements of head pose can be used to re-render the scene to match the end user's dynamically changing head location and orientation and provide an increased sense of immersion in the virtual space.
Head-worn displays that enable AR (i.e., the concurrent viewing of real and virtual elements) can have several different types of configurations. In one such configuration, often referred to as a “video see-through” display, a camera captures elements of a real scene, a computing system superimposes virtual elements onto the captured real scene, and a non-transparent display presents the composite image to the eyes. Another configuration is often referred to as an “optical see-through” display, in which the end user can see through transparent (or semi-transparent) elements in the display subsystem to view directly the light from real objects in the environment. The transparent element, often referred to as a “combiner,” superimposes light from the display over the end user's view of the real world.
VR and AR systems typically employ a display subsystem having a projection subsystem and a display surface positioned in front of the end user's field of view and on which the projection subsystem sequentially projects image frames. In true three-dimensional systems, the depth of the display surface can be controlled at frame rates or sub-frame rates. The projection subsystem may include one or more optical fibers into which light from one or more light sources emit light of different colors in defined patterns, and a scanning device that scans the optical fiber(s) in a predetermined pattern to create the image frames that sequentially displayed to the end user.
In one embodiment, the display subsystem includes one or more planar optical waveguides that are generally parallel to the field of view of the user, and into which light from the optical fiber(s) is injected. One or more linear diffraction gratings are embedded within the waveguide(s) to change the angle of incident light propagating along the waveguide(s). By changing the angle of light beyond the threshold of total internal reflection (TIR), the light escapes from one or more lateral faces of the waveguide(s). The linear diffraction grating(s) have a low diffraction efficiency, so only a fraction of the light energy is directed out of the waveguide(s), each time the light encounters the linear diffraction grating(s). By outcoupling the light at multiple locations along the grating(s), the exit pupil of the display subsystem is effectively increased. The display subsystem may further comprise one or more collimation elements that collimate light coming from the optical fiber(s), and an optical input apparatus that optically couples the collimated light to, or from, an edge of the waveguide(s).
With reference to FIG. 2, one embodiment of a display subsystem 20 comprises one or more light sources 22 that generates light, an optical fiber 24 that emits the light, and a collimation element 26 that collimates the light exiting the distal end of the optical fiber 24 into a light beam 36. The display subsystem 20 further comprises a piezoelectric element 28 to or in which the optical fiber 24 is mounted as a fixed-free flexible cantilever, and drive electronics 30 electrically coupled to the piezoelectric element 22 to activate electrically stimulate the piezoelectric element 28, thereby causing the distal end of the optical fiber 24 to vibrate in a pre-determined scan pattern that creates deflections 32 about a fulcrum 34, thereby scanning the collimated light beam 36 in accordance with the scan pattern.
The display subsystem 20 comprises a waveguide apparatus 38 that includes a planar optical waveguide 40 that is generally parallel to the field-of-view of the end user, a diffractive optical element (DOE) 42 associated with the planar optical waveguides 40, and in-coupling element (ICE) 42 (which take the form of a DOE) integrated within the end of the planar optical waveguide 40. The ICE 42 in-couples and redirects the collimated light 36 from the collimation element 26 into the planar optical waveguide 40. The collimated light beam 36 from the collimation element 26 propagates along the planar optical waveguide 40 and intersects with the DOE 42, causing a portion of the light to exit the face of the waveguide apparatus 38 as light rays 46 towards the eyes of the end user that are focused at a viewing distance depending on the lensing factor of the planar optical waveguide 40. Thus, the light source(s) 22 in conjunction with the drive electronics 30 generate image data encoded in the form of light that is spatially and/or temporally varying.
The location of each pixel visualized by the end user is highly dependent on the angle of the light rays 48 that exit the planar optical waveguide 40. Thus, light rays 48 that exit the waveguide 40 at different angles will create pixels at different locations in the field of view of the end user. For example, if it is desired to locate a pixel at the top right of the field of view of the end user, a collimated light beam 36 may be input into the waveguide apparatus 38 at one angle, and if is desired to locate a pixel at the center of the field of view of the end user, the collimated light beam 36 may be input into the waveguide apparatus 38 at a second different angle. Thus, as the optical fiber 24 is being scanned in accordance with a scan pattern, the light beam 36 originating from the optical fiber 24 will be input into the waveguide apparatus 38 at different angles, thereby creating pixels at different locations in the field of view of the end user. Thus, the location of each pixel in the field of view of the end user is highly dependent on the angle of the light rays 48 exiting the planar optical waveguide 40, and thus, the locations of these pixels are encoded within the image data generated by the display subsystem 20.
Although the angle of the light beam 36 entering the waveguide apparatus 38, and thus, the angle of the light beam 36 entering the planar optical waveguide 40 will differ from the angles of the light rays 48 exiting the planar optical waveguide 40, the relationships between the angle of the light beam 36 entering the waveguide apparatus 38 and the angles of the light rays 48 exiting the planar optical waveguide 40 is well-known and predictable, and thus, the angles of the light rays 48 exiting the planar optical waveguide 40 can be easily predicted from the angle of the collimated light beam 36 entering the waveguide apparatus 38.
It can be appreciated from the foregoing that the actual angles of the light beams 36 entering the waveguide apparatus 38 from the optical fiber 24, and thus, the actual angles of the light rays 48 exiting the waveguide 40 towards the end user be identical or near identical or one-to-one in relationship to the designed angles of the exiting light rays 48, such that the locations of the pixels visualized by the end user are properly encoded in the image data generated by the display subsystem 20. However, due to manufacturing tolerances between different scanners, as well, as changing environmental conditions, such as variations in temperature that may change the consistency of bonding materials used to integrate the display subsystem 20 together, the actual angles of the exiting light rays 48, without compensation, will vary from the designed angles of the exiting light rays 48, thereby creating pixels that are in the incorrect locations within the field of view of the end user, resulting in image distortion.
There, thus, is a need to ensure that the actual angles of light rays exiting the waveguide of a display subsystem in a virtual reality or augmented reality environment are as close to identical to the designed angles encoded within the image data generated by the display subsystem.